Method and arrangement in a telecommunication system

ABSTRACT

A method in a User Equipment (UE) of an Evolved Packet System (EPS) establishes a security key (K_eNB) for protecting Radio Resource Control/User Plane (RRC/UP) traffic exchanged with a serving eNodeB. The method comprises sending a Non-Access Stratum (NAS) Service Request to a Mobility Management Entity (MME), the request indicating a NAS uplink sequence number (NAS_U_SEQ). The method further comprises receiving an indication of the NAS_U_SEQ of the NAS Service Request sent to the MME, back from the MME via the eNodeB. The method further comprises deriving the K_eNB from at least the received indication of the NAS_U_SEQ and from a stored Access Security Management Entity-key (K_ASME) shared with said MME.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/677,675 filed Jul. 9, 2010, which is a 371 of InternationalApplication No. PCT/SE2008/050591, filed May 20, 2008, which claimsbenefit of U.S. Provisional Application No. 60/972,955, filed Sep. 17,2007, the disclosures of which are fully incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to methods and arrangements in atelecommunication system, and in particular to a security solution inthe EPS (Evolved Packet System), i.e. the E-UTRAN (the Evolved UMTSTerrestrial Radio Access Network) and the EPC (Evolved Packet Corenetwork), for UE-triggered service requests. More specifically, thepresent invention relates a method and an arrangement in an MME(Mobility Management Entity) and in a UE (User Equipment) for an EPS(Evolved Packet System) of establishing a security key for protectingRRC/UP traffic.

BACKGROUND

In the EPS-architecture, the subscriber authentication is performedbetween a UE and an MME (Mobility Management Entity), and the MMEmanages e.g. the mobility, the UE-identities and the securityparameters. The basis for defining the security procedure in the EPS isa security key, K_ASME, which is shared between the MME and the UE, andestablished at the authentication of the UE. A functional entity of theEPS-architecture called the ASME (Access Security Management Entity) maye.g. be co-located with the MME, and the ASME receives and stores thesecurity key K_ASME derived from the CK/IK-keys confined in the homenetwork. From the security key, K_ASME, the ASME derives an NAS securitycontext used to protect the NAS signalling, i.e. the Non Access Stratumsignalling between the MME of the Evolved Packet Core (EPC) network anda UE. The NAS security context contains parameters for the encryptionand integrity protection of the NAS signalling, such as K_NAS_enc,K_NAS_int, as well as uplink and downlink sequence numbers, NAS_U_SEQand NAS_D_SEQ, and the sequence numbers are used to prevent replay ofold messages, as well as for input to the encryption and integrityprotection procedures. The ASME provides the MME with the NAS securitycontext, and one NAS security context is kept in the MME, and acorresponding NAS security context is kept in the UE, and the replayprotection, integrity protection and encryption are based on that thesequence numbers of the NAS security contexts of the MME and UE are notreused.

Preferably, the security context for the protection of the UP/RRCtraffic between a UE and the serving eNodeB (i.e. a radio base stationin an EPS-architecture) is also based on said security key, K_ASME. Theprocedure to establish the UP/RRC security context involves deriving akey called K_eNB, from which the encryption key K_eNB_UP_enc is derivedfor protecting the UP (User Plane), i.e. the end user data transferredover EPC and E-UTRAN, as well as the encryption key, K_eNB_RRC_enc, andthe integrity protection key, K_eNB_RRC_int, for protecting the RRC(Radio Resource Control).

FIG. 1 illustrates a conventional exemplary signalling flow for aUE-initiated IDLE to ACTIVE state transition in an EPS-architecture. AnIDLE UE is only known to the EPC (Evolved Packet Core) of the EPS, andno UP/RRC security context exists between the eNodeB and the UE. An UEin the ACTIVE state is known both in the EPC and in the EUTRAN, and aUP/RRC security context has been established for protection of theUP/RRC-traffic between the UE and its serving eNodeB.

FIG. 1 illustrates a UE 11, an eNodeB 12, an MME 13, a serving GW(Gateway) 14, a PDN Gateway 15, and the HSS (Home Subscriber Server) 16.The Serving Gateway 14 is the node of the EPC that terminates theinterface towards EUTRAN, and the PDN Gateway is the node of the EPCthat terminates the interface towards a PDN (Packet Data Network). If aUE accesses multiple PDNs, there may be multiple PDN Gateways for thatUE. In signal S1 and signal S2, the NAS Service Request is transparentlyforwarded from the UE to the MME, and the NAS Service Request isintegrity protected based on NAS_U_SEQ. In the optional signal S3, theUE is authenticated by the MME and the K_ASME is established, usingsubscriber data stored in the HSS (Home Subscriber Server), and the MMEsends the Initial Context Setup Request to the eNodeB, in S4. In thesignals S5 and S6, the eNodeB establishes the radio bearer with the UEand forwards uplink data, and returns an Initial Context SetupComplete-message to the MME in signal S7. In signal S8, the MME sends anupdate bearer request to the Serving GW, and the serving GW responds insignal S9.

In prior solutions, the derivation of the K_eNB by the UE and MME forthe RRC/UP security context is based e.g. on a NAS SERVICE ACCEPTmessage or other explicit information sent from the MME to the UE.However, as illustrated in the exemplary conventional EPS signallingflow in FIG. 1, the MME will normally not send any NAS SERVICE ACCEPTupon receiving a NAS SERVICE REQUEST from a UE in an EPS. Therefore, itwill not be possible to derive the K_eNB from the information in a NASSERVICE ACCEPT message.

According to an exemplary known solution, the K_eNB is derived by theMME from the K_ASME and the NAS_D_SEQ used by the MME in the NAS SERVICEACCEPT message, and the UE derives the same K_eNB by retrieving thesequence number, NAS_D_SEQ, from the NAS SERVICE ACCEPT message andperforming the same K_eNB derivation as the MME. The MME transfers theK_eNB to the eNodeB when it sets up the S1-connection to the eNodeB.However, a drawback with this known solution is that if no explicit NASSERVICE ACCEPT message is defined from the MME to the UE, as in theexemplary conventional EPS signalling flow in FIG. 1, it is not possiblefor the UE to derive the same K_eNB as the MME. Even though it istechnically possible for the UE to estimate a current NAS downlinksequence number, NAS_D_SEQ, this estimation could be erroneous, sincethe MME may have sent NAS messages that were lost and never received bythe UE. In such a case, the MME would have updated its NAS_D_SEQ,without the UE being aware of the updating, leading to an erroneousNAS_D_SEQ in the UE.

According to another exemplary known solution, the derivation of theK_eNB is based on a separate sequence number maintained specifically forthe derivation of the K_eNB, and this sequence number is explicitlysynchronized during the NAS Service Request procedure either by the UEsending it to the MME, or by the MME sending it to the UE. However, adrawback with this solution is the extra complexity of the separatesequence number, since it has to be maintained both in the UE and in theMME in order to prevent replay attacks.

SUMMARY

The object of the present invention is to address the problem outlinedabove, and this object and others are achieved by the method andarrangement according to the independent claims, and by the embodimentsaccording to the dependent claims

The basic idea of the present invention is that the K_eNB is derivedfrom the K_ASME and from the NAS_U_SEQ of the NAS SERVICE REQUESTmessage from the UE to the MME, thereby triggering the establishment ofa UP/RRC security context in the eNodeB.

It is an advantage of the present invention that no explicit downlinkNAS SERVICE ACCEPT message or sequence number from the MME to the UE isrequired, and that the replay protection functionality of the NASsecurity context is re-used in the RRC and UP security contexts.

According to one aspect, the invention provides a method in a MobilityManagement Entity, MME, of an Evolved Packet System, EPS, ofestablishing a security key, K_eNB, for protecting RRC/UP trafficbetween a User Equipment, UE, and an eNodeB serving the UE. The methodcomprises the steps of receiving an NAS Service Request from the UE, therequest indicating a NAS uplink sequence number, NAS_U_SEQ; Deriving thesecurity key, K_eNB, from at least said received NAS_U_SEQ and from astored Access Security Management Entity-key, K_ASME, shared with saidUE; and forwarding said derived K_eNB to the eNodeB serving said UE.

According to a second aspect, the invention provides a MobilityManagement Entity, MME, for an Evolved Packet System, EPS. The MME isarranged to establish a security key, K_eNB, for protection ofRRC/UP-traffic between a UE and an eNodeB serving the UE. The MMEcomprises means for receiving an NAS Service Request from the UE, therequest indicating a NAS uplink sequence number, NAS_U_SEQ; Means forderiving a K_eNB from at least said received NAS_U_SEQ and from a storedAccess Security Management Entity-key, K_ASME, shared with said UE, aswell as means for sending said derived K_eNB to the eNodeB serving saidUE.

The first and the second aspect further provides methods, as well ascorresponding means, according to which the MME may derive the K_eNBfrom the NAS_U_SEQ and the K_ASME using a Pseudo-Random Function, PRF.The MME may further reconstruct the full NAS uplink sequence numberNAS_U_SEQ from received low order bits and integrity check the NASService Request received from the UE. Additionally, the MME may returnan indication of the received NAS_U_SEQ to the UE, and the NAS_U_SEQ maybe included in the set-up message forwarding the K_eNB to the eNodeB.Thereby, the UE does not have to remember the NAS_U_SEQ sent to the MME.

According to a third aspect, the invention provides a method in a UserEquipment, UE, of an Evolved Packet System, EPS, of establishing asecurity key, K_eNB, for protecting RRC/UP traffic exchanged with aserving eNodeB. The method comprises the steps of sending an NAS ServiceRequest to a Mobility Management Entity, MME, the request indicating aNAS uplink sequence number, NAS_U_SEQ; and deriving the K_eNB from atleast said NAS_U_SEQ and from a stored Access Security ManagementEntity-key, K_ASME, shared with said MME.

According to a fourth aspect, the invention provides a User Equipment,UE, adapted for an Evolved Packet System, EPS. The UE is arranged toestablish a security key, K_eNB, for protecting RRC/UP-traffic exchangedwith a serving eNodeB. The UE comprises means for sending a NAS ServiceRequest to an MME, the request indicating a NAS uplink sequence number,NAS_U_SEQ, as well as means for deriving a K_eNB from at least saidNAS_U_SEQ, and from a stored Access Security Management Entity-key,K_ASME, shared with said MME.

The third and the fourth aspect further provides methods, as well ascorresponding means, according to which the UE may derive the K_eNB fromthe NAS_U_SEQ and the K_ASME using a Pseudo-Random Function, PRF, andintegrity protect the NAS Service Request sent to the MME. Additionally,the UE may store the NAS_U_SEQ of the NAS Service Request sent to theMME, or, alternatively, receive an indication of the NAS_U_SEQ of theNAS Service Request sent to the MME, back from the MME via the eNodeB.This alternative embodiment has the advantage that the UE does not haveto remember the NAS_U_SEQ sent to the MME. The UE may further derive theK_eNB from the NAS_U_SEQ and the K_ASME after the reception of asecurity configuration message from the eNodeB.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, and withreference to the accompanying drawings, in which:

FIG. 1 is signalling diagram illustrating a conventional UE triggeredService Request in an EPS;

FIG. 2 is a signalling diagram illustrating the first embodiment of thisinvention, according to which the UE remembers the NAS_U_SEQ sent to theMME in an NAS SERVICE REQUEST-message;

FIG. 3 is a flow diagram illustrating the derivation of the K_eNB by theUE and the MME;

FIG. 4 is a signalling diagram illustrating a second embodiment of thisinvention, in which the MME returns the received NAS_U_SEQ to the UE;

FIG. 5 is a flow diagram illustrating the second embodiment depicted inthe FIG. 4; and

FIG. 6 a illustrates schematically an MME (Mobility Management Entity),and the FIG. 6 b illustrates schematically a UE, both provided withmeans for deriving the security key K_eNB.

DETAILED DESCRIPTION

In the following description, specific details are set forth, such as aparticular architecture and sequences of steps in order to provide athorough understanding of the present invention. However, it is apparentto a person skilled in the art that the present invention may bepractised in other embodiments that may depart from these specificdetails.

Moreover, it is apparent that the described functions may be implementedusing software functioning in conjunction with a programmedmicroprocessor or a general purpose computer, and/or using anapplication-specific integrated circuit. Where the invention isdescribed in the form of a method, the invention may also be embodied ina computer program product, as well as in a system comprising a computerprocessor and a memory, wherein the memory is encoded with one or moreprograms that may perform the described functions.

The concept or the invention is that the security key, K_eNB, is derivedfrom the Access Security Management Entity-key, K_ASME, and from theuplink sequence counter, NAS_U_SEQ, of the NAS SERVICE REQUEST messagesent from the UE to the MME, thereby triggering the establishment of theUP/RRC security context in the eNodeB.

When the UE is in IDLE mode, a NAS security context exists and comprisese.g. the above described K_NAS_enc, K_NAS_int, NAS_U_SEQ and NAS_D_SEQ,and the NAS messages are integrity—and possibly confidentialityprotected. The NAS security context thus also contains the securitycapabilities of the UE, in particular the encryption and integrityalgorithms.

The protection of the NAS messages is based on the NAS security keys,K_NAS_enc, K_NAS_int, and the uplink and downlink sequence counters,NAS_U_SEQ or NAS_D_SEQ, for the direction of the message. The fullsequence counter is normally not transmitted with the NAS message, onlysome of the low order bits, and the full sequence number will bereconstructed at the receiving end from a local estimate of the highorder bits and the received low order bits.

The concept of the invention may be explained in the context of thesignalling diagram for UE-triggered service requests, as depicted in theabove-described FIG. 1:

In S1 and S2 of the conventional signalling diagram in FIG. 1, a NASSERVICE REQUEST, comprising an uplink sequence counter, NAS_U_SEQ, isforwarded from the UE to the MME, and the NAS SERVICE REQUEST-message isintegrity protected based on said NAS_U_SEQ. The MME checks theintegrity of the message and accepts it if it is not a replay, and thisguarantees that the NAS_U_SEQ is fresh and has not been used before.

Thereafter, according to this invention, the MME derives the K_eNB basedat least on the received uplink sequence counter NAS_U_SEQ and on theK_ASME, using a conventional key derivation function, and this is notincluded in the conventional signalling diagram illustrated en FIG. 1.Consequently, the sequence counter may only be reset at theauthentication. The MME will send the derived K_eNB down to the eNodeBin, or piggybacked to, the message of the signal S4, the Initial ContextSetup Request (S1-AP).

In signal S5, the eNodeB sends a Radio Bearer Establishment and asecurity configuration message (Security Mode Command) to the UE. Thesemessages may be sent as two separate messages or combined in onemessage, as in FIG. 1, and the reception of these messages by the UEwill implicitly be a confirmation of the UEs NAS SERVICE REQUEST, insignal S1. The Security Mode Command will determine e.g. when theprotection should start and which algorithm to use.

According to this invention, the UE derives the K_eNB based at least onthe NAS_U_SEQ and the K_ASME, using a conventional key derivationfunction, upon reception of the message in signal S5, if not performedbefore. Thereafter, the eNodeB and the UE will establish the UP/RRCsecurity contexts, and this is not illustrated in the conventionalsignalling diagram in FIG. 1.

According to a first embodiment of this invention, the UE stores theuplink sequence counter, NAS_U_SEQ, included in the initial NAS SERVICEREQUEST in signal S1, and uses the stored NAS_U_SEQ for the derivationof the K_eNB.

However, according to a second embodiment, the MME includes the uplinksequence counter, NAS_U_SEQ, or only low order bits indicating theNAS_U_SEQ, in the S1-AP set-up message, in signal S4, sent to theeNodeB, in which case this information is also forwarded to the UE fromthe eNodeB during the RRC/UP context establishment. In this case, the UEwill be able to retrieve the indication of the NAS_U_SEQ from the eNodeBfor the derivation of the K_eNB, and does not have to keep the NAS_U_SEQof the NAS NAS SERVICE REQUEST-message sent to the MME in the signals S1and S2.

FIG. 2 illustrates the first embodiment of this invention, in which theUE keeps the NAS_U_SEQ of the initial NAS SERVICE REQUEST-message, inthe signal S21, for the derivation of K_eNB in the signal S24. The MMEwill receive the NAS_U_SEQ from the UE in the signal S21, or only loworder bits indicating the NAS_U_SEQ, and derive the K_eNB based on theNAS_U_SEQ and the K_ASME in S22. The MME forwards the derived K_eNB tothe eNodeB in the signal S23.

Thereafter, not illustrated in FIG. 2, the eNodeB and the UE willestablish the UP/RRC security context using the K_eNB, the UP/RRCsecurity contexts comprising the encryption key, K_eNB_UP_enc forprotecting the UP-traffic, as well as the encryption key and theintegrity protection key, K_eNB_RRC_enc and K_eNB_RRC_int, respectively,for protecting the RRC-traffic, thereby enabling a secure UP/RRCtraffic, in signal S25.

The derivation of the K_eNB is performed by a conventional keyderivation function, e.g. by a Pseudo-Random Function; K_eNB=PRF(K_ASME,NAS_U_SEQ, . . . ).

Further, as illustrated by the dots in the above-described PRF-function,the K_eNB-deriving function may have additional conventional inputvalues, such as e.g. the eNodeB-identity.

FIG. 3 is a flow diagram illustrating the method according to thisinvention, and in step 31, the UE 11 sends the initial NAS SERVICEREQUEST-message to the MME 13, the message indicating the NAS uplinksequence counter, NAS_U_SEQ, normally only by sending the low order bitsof the counter. In step 32, the MME receives the NAS SERVICEREQUEST-message from the UE, obtaining the NAS_U_SEQ, and reconstructingthe full sequence from the received low order bits. In step 33, the MMEderives the security key, K_eNB, from at least the received NAS_U_SEQand the K_ASME from the ASME (Access Security Mobility Entity), using asuitable key derivation function, e.g. a Pseudo-Random Function.

Thereafter, the MME forwards the derived K_eNB to the eNodeB 12, in step34, to be used by the eNodeB to establish the complete UP/RRC securitycontext shared with the UE. In step 35, said UE will derive the sameK_eNB from at least the stored K_ASME and from the NAS_U_SEQ of theinitial NAS SERVICE REQUEST-message transmitted from the UE to the MMEin step 31, and establish the UP/RRC security context from the derivedK_eNB.

In the first embodiment of this invention, the UE stores the NAS_U_SEQtransmitted to the MME in the initial NAS NAS SERVICE REQUEST-message,and uses the stored sequence number to derive the K_eNB.

FIG. 4 is a signalling diagram illustrating a second embodiment of thisinvention, in which the UE does not have to store the NAS_U_SEQ.Instead, the MME will return an indication of the received NAS_U_SEQback to the UE, via the eNodeB. In signal S41, corresponding to thesignal S21 in FIG. 2, the UE 11 transmits an initial NAS SERVICE REQUESTto the MME 13, indicating an uplink sequence number, NAS_U_SEQ, and MMEwill receive the NAS_U_SEQ and derive the K_eNB based on at least theNAS_U_SEQ and the K_ASME, in S42. However, according to this secondembodiment, the MME will include an indication of said receivedNAS_U_SEQ in signal S43 transmitted to the eNodeB 12 together with thederived K_eNB, and the eNodeB will forward the NAS_U_SEQ to the UE, insignal S44. Thereafter, the UE will derive the K_eNB from at least theK_ASME and from the NAS_U_SEQ returned by the MME, in signal S45. Fromthe derived security key, K_eNB, the eNodeB and the UE will establishthe UP/RRC security context, thereby enabling secure UP/RRC traffic, insignal S46.

FIG. 5 is a flow diagram illustrating the above-described methodaccording to a second embodiment of this invention, in which anindication of the NAS_U_SEQ is returned to the UE by the MME. In step41, the UE 11 sends the initial NAS SERVICE REQUEST-message to the MME13, the message indicating the NAS uplink sequence counter, NAS_U_SEQ,normally the low order bits. In step 52, the MME receives the NASSERVICE REQUEST-message from the UE, thereby obtaining the NAS_U_SEQ,and, if necessary, reconstructing the full NAS_U_SEQ from the receivedlow order bits. In step 53, the MME derives the security key, K_eNB,from at least the received NAS_U_SEQ and the K_ASME, using a suitablekey derivation function.

Thereafter, the MME includes an indication of the NAS uplink sequencecounter, NAS_U_SEQ, in the message forwarding the derived K_eNB to theeNodeB 12, in step 54, and the eNodeB uses the received security key,K_eNB, for establishing a UP/RRC security context. The receivedNAS_U_SEQ is forwarded to the UE 11 by the eNodeB, in step 55, and instep 56, the UE derives the security key, K_eNB, from at least theK_ASME and from said received NAS_U_SEQ, in order to establish theUP/RRC security context shared with the eNodeB.

The derivation of the K_eNB by the MME, in the step 53, and by the UE,in the step 56, is performed by a suitable conventional key derivationfunction, e.g. a Pseudo-Random Function; K_eNB=PRF(K_ASME, NAS_U_SEQ, .. . ). Normally, the key derivation function will have additionalconventional input values, e.g. the eNodeB-identity.

FIG. 6 a illustrates an MME 13 (Mobility Management Entity) for an EPS,according to this invention, further arranged to establish a securitykey, K_eNB, for a security context for the protection of UP/RRC trafficbetween a UE and a serving eNodeB. The MME is provided with conventionalcommunication means, not illustrated in the figure, for communicatingwith the nodes in the EPS, e.g. with the eNodeBs via an S1-MMEinterface. Further, in the MME of FIG. 1, an ASME (Access SecurityManagement Entity) 61 is illustrated by hatched lines, since thisfunctional entity of an EPS may be co-located with the MME.

The means of the MME 13 illustrated in FIG. 6 a for establishing thesecurity key, K_eNB, comprises reception means 62 for receiving an NASSERVICE REQUEST-message including an NAS_U-SEQ from a UE (via itsserving eNodeB; key derivation means 63 for deriving a security key,K_eNB based on at least the received NAS_U-SEQ and a stored K_ASME,using a conventional key derivation function; and sending means 64 forsending the derived K_eNB to the eNodeB serving the UE.

FIG. 6 b illustrates a UE 11 (User Entity) according to this invention,the UE adapted for an EPS, and further arranged to establish a securitykey, K_eNB, for a security context for the protection of UP/RRC trafficexchanged with its serving eNodeB. The UE is provided with conventionalcommunication means, not illustrated in the figure, for communicatingwith the nodes in the EPS via an LTE-Uu interface to its serving eNodeB.

The means of the UE 11 illustrated in the FIG. 6 b for establishing thesecurity key, K_eNB, comprises sending means 66 for sending an NASSERVICE REQUEST-message to the MME, via the serving eNodeB, the requestindicating an uplink sequence number, NAS_U-SEQ, and the means forestablishing a security key, K_eNB, comprises key derivation means 67for deriving a security key, K_eNB based on at least the NAS_U-SEQ and astored K_ASME, using a conventional key derivation function.

The above-described means of the MME and the UE, as illustrated in theFIGS. 6 a and 6 b, implement the described functions using a suitablecombination of software and hardware, e.g. a programmed microprocessoror an application-specific integrated circuit, as well as conventionalradio transmitters and receivers.

While the invention has been described with reference to specificexemplary embodiments, the description is in general only intended toillustrate the inventive concept and should not be taken as limiting thescope of the invention.

1. A method in a User Equipment (UE) of an Evolved Packet System (EPS)of establishing a security key (K_eNB) for protecting Radio ResourceControl/User Plane (RRC/UP) traffic exchanged with a serving eNodeB, themethod comprising: sending a Non-Access Stratum (NAS) Service Request toa Mobility Management Entity (MME), the request indicating a NAS uplinksequence number (NAS_U_SEQ); receiving an indication of the NAS_U_SEQ ofthe NAS Service Request sent to the MME, back from the MME via theeNodeB; deriving the K_eNB from at least the received indication of theNAS_U_SEQ and from a stored Access Security Management Entity-key(K_ASME) shared with said MME.
 2. The method in a UE according to claim1, wherein the K_eNB is derived from the NAS_U_SEQ and the K_ASME usinga Pseudo-Random Function (PRF).
 3. The method in a UE according to claim1, further comprising integrity protecting the NAS Service Request sentto the MME.
 4. The method in a UE according to claim 1, wherein theK_eNB is derived from the NAS_U_SEQ and the K_ASME after the receptionof a security configuration message from the eNodeB.
 5. A user equipment(UE) adapted for an Evolved Packet System (EPS), the UE arranged toestablish a security key (K_eNB) for protecting Radio ResourceControl/User Plane (RRC/UP) traffic exchanged with a serving eNodeB, theUE comprising: a transmitter configured to send a Non-Access Stratum(NAS) Service Request to a Mobility Management Entity (MME), the requestindicating a NAS uplink sequence number (NAS_U_SEQ); a receiverconfigured to receive an indication of the NAS_U_SEQ of the NAS ServiceRequest sent to the MME, back from the MME, via the eNodeB; and aprocessor configured to derive a K_eNB from at least the receivedindication of the NAS_U_SEQ, and from a stored Access SecurityManagement Entity-key (K_ASME) shared with said MME.
 6. The UE accordingto claim 5, arranged to derive the K_eNB from the NAS_U_SEQ and theK_ASME by using a Pseudo-Random Function (PRF).
 7. The UE according toclaim 5, arranged to integrity protect the NAS Service Request sent tothe MME.
 8. The UE according to claim 5, arranged to derive the K_eNBafter the reception of a security configuration message from the eNodeB.